The idea behind a Roth IRA is taxes will go up in the future so you are best off paying less in taxes now than in the future, which is why Roth IRAs are contributed to with post-tax dollars whereas traditional IRAs are contributed to with pre-tax dollars. The theoritical advantage comes when you want to withdrawal your money. With the traditional IRA, when you withdrawal money, you pay ordinary income tax on all withdrawals. With a Roth IRA, all withdrawals (after the age of 59 1/2) are tax free, including any gains you may have made.
To illistrate, with a very simple example, assume you make $50,000 and your IRA grows at 5% for 40 years.
Traditional IRA - $5,000
Roth IRA - $3,750 ($5,000 after taxes)
Totals after 40 years
Traditional IRA - $604,000
Roth IRA - $453,000
Withdrawal equally over 15 years
Traditional IRA - $604,000 / 15 = $40,266 * 75% (25% tax) = $30,200 / year
Roth IRA - $453,000 / 15 = $30,200/ year
First, this was not a contrived example and I was surprised the numbers worked out this way. Second, as you can see with this example there is really no advantage either way unless you by into the theory of higher taxes in the future.